1. Field of the Invention
This invention is directed to methods for enhancing the adhesion of composite electrodes onto metal foils (i.e., current collectors), to metal foils prepared by these methods and to electrolytic cells produced from these metal foils.
2. State of the Art
Electrolytic cells comprise a cathode, an anode and an electrolyte interposed therebetween. Electrolytic cells are often defined as liquid or solid cells and this refers merely to whether the electrolyte interposed between the anode and the cathode is a liquid or a solid. Solid electrolytic cells are well known in the art and present many advantages over conventional liquid batteries such as improved safety features, lighter weight, etc.
In order to enhance the overall current produced by solid or liquid batteries, it is conventional to employ several electrolytic cells in a battery. When so employed, the current from each of the cells is accumulated so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery.
One method for accumulating the current from individual electrolytic cells is by using a current collector attached to the cathode or the anode of the electrolytic cell. Typically, the current collector is a metal foil which is coupled to other current collectors in the battery so that the current generated by each cell is collected and accumulated over all of the cells. Thus, the total current generated by the battery is approximately a summation of the current generated by each of the electrolytic cells employed in the battery.
Notwithstanding the benefits of using current collectors (i.e., metal foils) in electrolytic cells, there is a problem when current collectors are used in conjunction with composite electrodes (i.e., electrodes derived from a composite of different materials). Specifically, composite electrodes are typically prepared from a paste which is applied onto the surface of the current collector and then cured to form the electrode.
For example, one conventional method for forming a composite cathode is by first forming a cathode paste comprising a compatible cathodic material (e.g., LiV.sub.3 O.sub.8, V.sub.6 0.sub.13 and the like), a conductive material such as carbon powder, an electrolytic solvent, and a prepolymer or a mixture of prepolymers. The cathode paste is then coated at a set thickness by extrusion or other suitable application means onto a metal foil which acts as a current collector and the paste is then cured to provide for a solid cathode having a current collector attached to one of its surfaces.
However, the metal foils used as current collectors (e.g., aluminum foil, nickel foil, etc.) typically lack adequate adherence so as to permit a paste to adequately adhere to the foil. Thus, during application of a paste onto the foil, the paste can become dislodged from the foil. In turn, after curing, such dislodgement will result in defects in collecting current from that cell.
One method disclosed in European Patent Application Publication No. 0 397 523 ("EPA '523") for overcoming the inadequate adherence of metal foils to an electrode paste is to roughen the surface of the metal foil with electrolytically deposited metal. The electrolytically deposited metal "roughens" the surface of the foil and one commercially employed current collector is a roughened nickel on nickel current collector which is available as CF18/NiT from Fukuda Metal Foil & Powder Company, Ltd., Kyoto, Japan. The electrolytic deposition process results in the formation of a roughened surface characterized on a microscopic level by peaks and valleys rather than a smooth planar surface (see FIGS. 3, 4 and 6 of EPA '523). Such roughened surfaces lead to a more adherent surface.
On the other hand, roughened metal on metal foils are rather expensive and result in undesirable weight increases for the metal foil current collector which translates into an undesirable increased weight for electrolytic cell itself. Moreover, electrolytic deposition of metal is not practical for certain metal foils. For example, aluminum foil typically contains an outer passivation layer of aluminum oxide which inhibits the electrolytic deposit of aluminum onto the surface of the aluminum foil.
Another common method in the art for coating a metal foil with carbon is to paint the metal foil with a carbon-containing paint. After painting, the solvent in the paint has to be dried out before the coated foil can be used. Thus, this type of process has the disadvantage of being slow and also of being difficult to consistently apply an even layer of electrically-conducting, adhesion promoter over the surface of the metal foil.